1. Field of the Invention
The present invention relates to a coordinate measuring device and, more particularly, to a laser based coordinate measuring device.
2. Discussion of the Related Art
There is a class of instrument that measures the coordinates of a point by sending a laser beam to a retroreflector target that is in contact with the point. The instrument determines coordinates by measuring the distance and the two angles to the retroreflector target. There is another class of instrument that is capable of measuring the three orientation angles (pitch, yaw, and roll) of a retroreflector target. If such an instrument can also measure the three coordinates of a point in space, it is said to measure six degrees of freedom. However, such six degree-of-freedom systems, whether or not they are employing laser techniques, are generally inaccurate, slow, limited in radial or angular range, and/or expensive. Exemplary systems for determining position (three to six degrees of freedom) are described by U.S. Pat. No. 4,790,651 to Brown et al.; U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No. 5,059,789 to Salcudean; U.S. Pat. No. 5,367,373 to Busch-Vishniac et al.; U.S. Pat. No. 5,973,788 to Pettersen et al.; and U.S. Pat. No. 5,267,014 to Prenninger, et al. (the disclosures of which are hereby incorporated by reference).
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. To provide a beam-steering mechanism for this tracking function, laser trackers conventionally include a stationary base onto which a rotating stage or platform is mounted. Until now, most laser trackers have used optical elements, such as mirrors or prisms, to steer the laser beam from its source in the base to optics in the rotating stage and through or off those optics toward the retroreflector. These optical elements and their mounts are costly. Also, they are subject to tilting and bending as a result of thermal and/or mechanical stresses that are usually present in tracker work environments. The consequence of these stresses is reduced accuracy and stability. Examples of beam-steering laser trackers are described by Lucy, et al., Applied Optics, pp. 517-524, 1966; Bernard and Fencil, Applied Optics, pp. 497-505, 1966; Sullivan, SPIE, Vol. 227, pp. 148-161, 1980; U.S. Pat. No. 4,020,340 to Cooke; U.S. Pat. No. 4,025,193 to Pond; U.S. Pat. No. 4,386,848 to Clendenin et al.; U.S. Pat. No. 4,436,417 to Hutchin; U.S. Pat. No. 4,457,625 to Greenleaf et al.; U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No. 4,721,385 to Jelalian et al.; Gennan Patent DE 3205362 A1 to Pfeifer et al. (which are hereby incorporated by reference). An example of a beam-steering mechanism that uses prismatic optical elements is described by U.S. Pat. No. 4,790,651 Brown et al. (which is hereby incorporated by reference).
A device that is closely related to a laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. This broad definition of laser tracker, which includes laser scanners, is used throughout this application.
An alternative to steering the laser beam with a mirror or prism is to launch the laser beam from an optical fiber mounted on a rigid platform. Although such devices have been built, none has taken full advantage of the simplicity, stability, and flexibility possible with such an approach. For example, such systems usually require separate optical fibers for transmitting and receiving the laser light. An exemplary system that tracks a laser beam launched from an optical fiber is described in Nakamura, et al., Review of Scientific Instruments, pp. 1006-1011, 1994; Takatsuji et al., easurement Science & Technology, pp. 38-41, 1998; Takatsuji, et al., Measurement Science & Technology, pp. 1357-1359, 1998; and Takatsuji, et al., Dimensional Metrology in the 21st Century, International Dimensional Metrology Workshop sponsored by Oak Ridge Metrology Center, May 10-13, 1999 (which are hereby incorporated by reference). Non-tracking systems that launch laser beams from optical fibers are numerous in the prior art and include U.S. Pat. No. 4,459,022 to Morey; U.S. Pat. No. 5,095,472 to Uchino, et al.; U.S. Pat. No. 5,198,874 to Bell et al.; U.S. Pat. No. 5,200,838 to Nudelman; U.S. Pat. No. 5,402,230 to Tian, et al.; U.S. Pat. No. 5,508,804 to Furstenau; and U.S. Pat. No. 5,557,406 to Taylor (which are hereby incorporated by reference).